The present invention relates to persistent current, thermally controlled superconductive switches for use in high energy superconductive magnets and more particularly to superconductive switches for use in conduction cooled superconductive magnets.
Superconductive switches are used to turn superconductive magnet circuits on and off. A superconductive magnet energized in a true superconductive state has no electrical current losses provided there are no resistive components in the circuit. During the ramping of superconductive magnets, a power supply is attached to the magnet through current leads. The inability of these leads to be superconducting between the power supply, which is at room temperature, and the magnet, which is at a temperature of a few Kelvin, imposes electrical resistances within the magnet's electrical circuit and prevents the magnet from being persistent. A superconductive switch is used to bypass the resistive parts of the circuit thereby initiating a truly persistent mode of operation. The switch includes superconductive windings which are in a normal state (resistive) during current ramping of the magnet and in a superconductive state during the persistent mode of operation after ramping is completed. The switching occurs by powering a heater attached to the superconductive switch, causing the superconductor in the switch to increase in temperature beyond the transition temperature of the superconductor introducing a finite resistance to the flow of current to turn the switch "off". The persistent mode of operation is initiated by allowing the superconductive switch which is carrying current to cool below its transition temperature.
Superconductive switches are fabricated from as many turns of superconductive wire as are necessary to achieve the desired resistance in the "off" state and to provide sufficient heat capacity to the switch to avoid damage when the switch is in the normal state. In prior switches employing copper matrix superconductor, a very long wire was required to achieve even a small resistance. During ramping, when a voltage is imposed across the superconductive switch which is in the normal mode and in parallel with the superconductive winding to be energized, a high resistance is desirable since the dissipation of thermal energy in the switch is inversely proportional to the normal state electrical resistance of the switch. A low resistance switch can cause substantial energy dissipation (V.sup.2 /R losses). In helium cooled superconductive magnets, the dissipated energy from the switch causes excess helium boil off. For a cryogen free conduction cooled magnet such as the one shown and claimed in U.S. Pat. No. 4,924,198, and hereby incorporated by reference, the switch's thermal energy will impose an extra load on the cryocooler and if excessive, can cause the magnet to quench.
In U.S. Pat. No. 4,904,970, a superconductive switch is shown and claimed which is wound with cupro nickel matrix niobium-titanium superconductor which has a high resistance in the normal state to reduce the amount of boil off during ramp up in a cryogen cooled superconductive magnet. Some of these high resistance switches have failed after several months of operation even though they are immersed in liquid helium during persistent mode operation.
Presently, cryogen free conduction cooled superconductive magnets are powered by a permanently connected stable power supply which provides the energy lost in the current leads to provide a constant current flow in the superconductive windings which is necessary to create a homogeneous field. Superconductive switches are not used. The conduction cooled magnet is typically cooled by a two stage cryocooler having a temperature of approximately 40K at the first stage and 10K at the second stage. The excess cooling capacity of second stage of the cryocooler which provides its cooling at 10K is limited and the cooling capacity is needed to remove conduction and radiation losses of the windings suspension.
It is an object of the present invention to provide a superconductive switch which can be used in conduction cooled superconductive magnets.
It is a further object of the present invention to provide a superconductive switch which uses tape superconductor and has a relatively high resistance when operated above the transition temperature of the superconductor.
It is a still further object of the present invention to provide a superconductive switch which does not need to be immersed in liquid cryogen during ramp up, shut down, or persistent operation when connected in parallel with a high energy magnet.
It is another object of the present invention to provide a superconductive switch which has a low thermal mass and a quick recovery time.
It is yet another object of the present invention to provide a superconductive switch which is stable in a magnetic field for long periods of time.